1. Microphononic Crystals B. Ash, G. R. Nash, P. Vukusic bja204@exeter.ac.uk ex.ac.uk/BenAsh www.exeter.ac.uk/metamaterials Abstract. This project investigates Phononic crystals (PnCs) for MHz surface acoustic waves (SAWs), guided by piezoelectric substrates with attention paid to new engineering methods, shape of structures and waveguide applications. Introduction PnCs exhibit acoustic stop bands through both Bragg interference and local resonances of phonons. PnCs for high frequency SAWs have garnered recent attention due to the potential applications in RF signal processing, with piezoelectric SAW devices being widely used as delay lines, filters etc. Figure (1) shows PnC developments in literature, (a) illustrates fabrication through RIE etching of void cylinders, with shape and depth defects resulting in attenuation due to bulk coupling. (b) illustrates the prospect of PnCs as waveguides using Finite Difference Time-Domain Method. Results Discussion References [1] S.Benchabane, A. Khelif, J.-Y.Rauch, L. Robert and V. Laude, Phys. Rev. E 73, 065601 (2006) [2] Jia-Hong Sun and Tsung-Tsong Wu, Phys. Rev. B 74, 174305 (2006) Figure 1 (a) shows SAW transmission spectra for PnC shown in inlayed SEM image; conical depth profile results in attenuation outside of theoretical stop band shaded [1]. (b) shows FDTD demonstration of PnC for SAW waveguide [2]; grayscale represents out of plane displacement field. (a) (b) 400μm (c) (b) 39.5MHz (a) 25MHz Arc length (mm) Surface displacement Z component x10 -8 [m] Arc length (mm) Surface displacement Z component x10 -9 [m] Figure 2. (a) and (b) show FEM simulated SAW propagation through a PnC array of annular void inclusions with a pitch over diameter ratio of 2 studies at in band and out of band frequencies respectively. (c) shows SEM image of preliminary fabricated annular hole structure PnC using Focused Ion Beam lithography. Focused Ion Beam lithography studied because commonly used alternatives result in lateral under-cutting and conical holes, as seen in Figure (1). Limitation of this method is time required, therefore novel fabrication concepts of an annular hole structure and large pitch to diameter ratios are being investigated to reduce volume etched. Finite Element Method investigations confirm simulated validity of these concepts as seen in Figure (2) (a-b) where out of band frequency SAWs experience progressive attenuation through the PnCs and in band frequencies do not. Preliminary PnC devices have been fabricated as shown in Figure (2) (c) to test Focussed Ion Beam lithography for this application and early SAW transmission measurements indicate successful PnC stop band behaviour from these. These devices also indicate unwanted in band attenuation at higher frequencies which may be due to limited depth of holes. Ongoing work includes depth profiling of etched holes to investigate this attenuation, optimisations of PnC array dimensions and setting up a stable vacuum environment for highly reproducible SAW transmission measurements.